ABSTRACT

The Smackover Formation is a major hydrocarbon source rock in the Gulf Coastal region. During the Cretaceous, the hydrocarbon-rich Smackover was intruded by numerous alkalic dikes which are suspected to have transferred heat to the surrounding host rocks and caused conversion of liquid hydrocarbons to gas. A drill core recovered from the Carter Hope-Fee Well, Morehouse Parish, LA records the presence of an 11 m section of intrusive alkaline igneous rock penetrating the Smackover siltstones and carbonate mudstones at 1.9 km depth. Surrounding the dike is an asymmetric contact metamorphic aureole as recorded, in part, by newly developed metamorphic/metasomatic minerals in the host rock four meters above and three meters below the dike. Adjacent to the dike, the siltstones and mudstones were transformed to produce metamorphic mineral assemblages rich in alkali and alkaline earth elements as well as fluorine; hydrogrossular, diopside, pectolite, apophyllite, fluorite, and feldspars. Modes of minerals change as a function of distance from the dike, thermal environment, and protolith composition. Formation of these mineral assemblages requires increased temperatures and significant mass transport of components from the dike into the host rocks.

Numerical heat transport calculations from an 11 m dike suggest temperatures near 400°C adjacent to the dike, consistent with temperatures calculated from mineral chemical data. However, this thermal mass produces an alteration halo much larger than that observed from the mineralogy or chemistry. Varying porosity, permeability, latent heat of crystallization and heat capacity within permissible values do not significantly alter the size of the alteration zone. In order to produce the small alteration halo, rapid heat removal must have accompanied dike emplacement. This can be accomplished if a two meter dike is emplaced nearly vertically such that fluid convection occurs and a thermal boundary layer develops which rapidly dissipates heat. This also requires that the 11 m section represents an apparent thickness.

The small size of the thermal alteration zone suggests that if gas is to be produced by thermogenic sources such as dikes, that many dikes must be emplaced within close spatial and temporal proximity. This study also demonstrates that significant fluid infiltration and mass transport accompany dike intrusion.